The present invention relates generally to flow control apparatus and to systems and methods employing the same which are used to separate fluids of differing densities, and more particularly, to equipment used to separate gases and liquids during the production and refining of hydrocarbons such as natural gas and oil.
Many fluid flow systems require the separation of fluids having components of differing densities. A prime example is in the production and refining of hydrocarbon liquids and gases. These production fluids often include natural gas, carbon dioxide, oil, water, nitrogen, hydrogen sulfide, and helium along with other fluid and solid contaminants. At some point, it is necessary to separate gases from liquids and water from oil in order to measure, transport, or process the hydrocarbon fluids. A significant shortcoming to most pipeline transport and separation systems is that they employ flow control apparatus which tend to shear and disperse coalesced droplets and stratified layers of fluid components when a fluid mixture passes through the flow control apparatus. This adversely affects the ability of a cooperating downstream separation apparatus to separate fluids of differing densities.
Initially, production fluids are withdrawn from wells drilled in the earth. The production fluids are typically transported to a gas separator where free gas is removed. The liquid then passes to an oil/water separator where most of the water is removed. Examples of conventional gas separators include horizontal and vertical gravity separators and gas/liquid cylindrical cyclones. Examples of conventional liquid separators include horizontal gravity separators, free water knock-outs, liquid/liquid hydrocyclones, and flotation devices.
Various flow control apparatus are used in these gas and liquid separation systems to control the flow of the production fluids. For example, production fluids may be produced from wells at very high pressures. Downstream processing equipment is generally not built robust enough to handle these high pressures in order that the processing equipment may be built economically. Consequently, pressure reducing chokes must be incorporated into the system between the well and downstream processing equipment. Control valves, check valves and other control apparatus are also used to control the flow rate of the production fluids from a well. Other examples of flow control apparatus include homogenizers, mixers, pumps, elbows, venturis, orifice plates, etc. Similarly, the processing of hydrocarbons in refineries often employs many of these same flow control apparatus.
There is a natural tendency for gravity to separate fluid components of differing densities and to concentrate fluids of similar densities, if the fluid flow is sufficiently quiet and given adequate residence time. Further, there is a tendency for droplets in a dispersed phase to coalesce given close enough proximity and adequate contact time for film drainage to remove the fluid barrier between droplets. Separation equipment which is employed to separate fluids of differing densities, such as water and oil, generally operate much more effectively if dispersed droplets in the incoming fluids are large, able to coalesce, stratify and pre-separate prior to entering the separation equipment.
However, the use of conventional flow control apparatus in these separation systems tends to shear and disperse droplets and destratify layers of separated components. Mechanically, this occurs because these flow control apparatus are typically designed such that there is a rapid change in both the flow rate and direction of a fluid mixture passing through the flow control apparatus with energy being dissipated into the fluid. As the rate of energy dissipation per unit volume is increased, smaller droplets are generally created. The shear forces induced during passage through these conventional flow control apparatus tend to tear apart and disperse any stratified layers of fluid which have formed and also disperse large clumps or droplets of one fluid component into another. Likewise, in severe situations, coalesced droplets of oil and water may also be broken up into tiny or microscopic droplets and dispersed under the shear stresses imparted by their passage through these flow control apparatus. Consequently, fluid passage through conventional flow control apparatus often results in the breakup and dispersion of separated layers and coalesced droplets and even in the formation of emulsions. According to Stokes Law, the velocity of a droplet of one fluid falling or rising through another is proportional to the droplet size. Thus, the use of these conventional flow control apparatus in separation systems may be counterproductive to the end goal of producing separated fluids.
Another drawback to conventional flow conditioning equipment is that they are highly susceptible to erosion and wear. Particles, such as sand, which impact components at high velocities and generally perpendicular to a surface, can cause significant wear on the equipment. It would be desirable to extend the life of such equipment by reducing this erosion and wear.
As a specific example, conventional chokes, used to provide pressure letdown, are notorious for breaking up droplets, increasing phase dispersion, worsening emulsions, and eroding in the presence of sand. The extent to which a choke can worsen fluid separation is difficult to predict in advance. Therefore, separation apparatus are often grossly oversized to compensate for the uncertainty of the dispersion effect of the choke or, worse, undersized if the effect of the choke is not adequately accounted for. If dispersion of coalesced droplets is sufficiently severe, chemicals such as deemulsifiers may have to be added to the water and oil mixture to assist in the separation process. Further, in some instances, heat may have to be added to enhance separation. Moreover, these separation apparatus may be mounted in remote areas such as on the sea floor or on an offshore platform where size and weight are important. Consequently, it is desirable to keep separation apparatus as small and light in weight as possible while still achieving a desired level of separation.
Accordingly, there is a need for flow control apparatus which work in cooperation with downstream separation apparatus to minimize the shearing or breaking up of oil layers and droplets in an oil and water mixture during hydrocarbon production and processing. Similarly, other industries, which use flow control apparatus like those described above to separate components in a fluid mixture, also face comparable problems. The present invention reduces the aforementioned shortcomings of many of these separation systems employing conventional flow control apparatus, and in particular, in those systems used in the processing of hydrocarbons.
The present invention includes a mechanical flow conditioning technology for the purpose of improving downstream separation of oil, water and gas. The technology involved is based on the concepts of reducing the forces that break up droplets, and swirling the bulk flow to enhance coalescence of the dispersed phase. Centrifugal forces in the swirling flow field segregate fluid components according to density and cause droplets to crowd together allowing coalescence of multiple droplets into larger droplets. According to Stokes law, droplets with larger diameters will move through a continuous fluid faster and will consequently separate more quickly. Incorporating this technology can result in improved performance from existing separators or allow the use of smaller separators to perform the same duty. Such minimization of separator size is quite desirable when a separator is used in offshore or sea floor separation settings where size and weight reduction are at a premium.
A xe2x80x9ccoalescing or flow conditioning chokexe2x80x9d design is disclosed which produces a pressure drop through a combination of series and parallel swirl producing components. Droplet size is inversely proportional to the square of impact velocity. Impact velocity is the relative velocity between impacting fluids or between a fluid and a wall. The coalescing choke design of the present invention keeps this impact velocity small by orienting pressure dissipating orifices to direct fluid passing therethrough to swirl helically and along the inner periphery of a receiving chamber. Accumulating pressure losses are achieved through a series of successive orifices or other flow restrictions rather than taking one large loss through a single opening as is typical of conventional chokes. This gradual, as opposed to abrupt, pressure drop through orifices reduces the rate of energy dissipation per unit volume which helps maintain droplets in a coalesced state or at least minimizes breakup and dispersion. Further, the strong fluid rotation produced by this configuration reduces the relative velocity differential between droplets or stratified layers of incoming fluid and generates a centrifugal field, which can greatly enhance droplet coalescence. Such a flow control apparatus has been demonstrated to significantly reduce the time required to separate oil and water in a downstream separator as compared to using a similar non-coalescing choke design in a like separation system
This principle of minimizing velocity differentials between fluid components and maximizing centrifugal forces in a swirl chamber can be incorporated into the design of other devices, e.g., control valves, swirl vanes, piping elbows and fittings, to enhance coalescence and improve performance of downstream separation apparatus.
A flow conditioning and separation system for separating liquid components of differing densities from a fluid mixture is disclosed. The system comprises a flow conditioning apparatus and a cooperating liquid separation apparatus disposed downstream from and in fluid communication with the flow conditioning apparatus. The flow conditioning apparatus has an inlet, an outlet, and a swirl chamber extending along a curvilinear swirl axis. The inlet and outlet are configured to cooperate with the swirl chamber to induce the swirling of a fluid mixture about the swirl axis such that when a fluid mixture having liquid components of differing densities passes through the swirl chamber, centrifugal forces are imparted upon the liquid components to enhance coalescence or at least minimize dispersion of droplets in at least one of the liquid components. The liquid separation apparatus is capable of separating liquids of differing densities. The enhanced quantity of coalesced droplets in a fluid mixture received from the cooperating upstream flow conditioning apparatus by the separation apparatus increases the separation efficiency of the separation system over a system which does not use a flow conditioning apparatus.
Ideally, the inlet and the outlet direct fluid to flow generally circumferentially within the swirl chamber to create a helical swirling motion about the swirl axis. At least one of the inlet and the outlet may include a plurality of orifices which have peripheries which are elongate and curved and allow a fluid mixture to pass therethrough directed generally in a circumferential direction relative to the swirl axis.
The flow control apparatus may serve as a choke to reduce pressure, a flow control valve to control the rate of flow through the flow control apparatus or else as an elbow to help redirect the direction of flow. The inlet and outlet may include a plurality of orifices in series and/or in parallel. Further, a movable closure in the flow conditioning apparatus may be used to control flow rate. Moreover, methods employing such flow control apparatus to separate fluid components of differing densities in a separation system are also within the scope of the present invention.
It is an object of the present invention to provide a separation system which is compact in size and low in weight, yet is efficient in separating fluid components of differing densities by employing a flow conditioning apparatus in the separation system upstream from a cooperating separation apparatus.
It is another object to increase the efficiency of separation systems by employing flow control apparatus which preferably enhance the coalescence, or at least minimize the dispersion, of droplets of liquids passing through the flow control apparatus before reaching a cooperating separation apparatus which separates fluids of differing densities.
It is yet another object to provide a flow conditioning apparatus which includes an inlet, an outlet, and a swirl chamber which are configured to induce a fluid mixture to swirl, preferably helically, when passing through the swirl chamber to impart centrifugal forces on fluid components of differing densities thereby enhancing coalescence of droplets and stratification of layers of the fluid mixture.
An additional object is to provide a flow conditioning apparatus which includes an inlet, an outlet and a swirl chamber wherein the inlet and the outlet are configured to direct fluid flow generally tangential to the surface enclosed by the swirl chamber, thereby minimizing the rate of change of direction of fluid flow and relative velocity differentials between droplets and stratified layers of fluid components passing through the flow control apparatus.
It is still a further object to provide fluid conditioning apparatus which minimizes the maximum velocity of particles flowing through fluid conditioning equipment to thus reduce wear and extend the life of the equipment.
An additional object is to provide a flow conditioning choke apparatus which significantly reduces the pressure of fluid passing therethrough while minimizing the shearing of fluids to maximize the size of droplets of immiscible fluid components exiting the flow conditioning choke apparatus.
Moreover, it is an object to provide a system for separating hydrocarbons from water in a separation system where an upstream flow conditioning apparatus minimizes fluid shear to enhance droplet size and stratification of layers of fluids of differing densities such that a downstream separation apparatus may more effectively separate the water from the hydrocarbons and be made of a minimum weight and of a minimum footprint.